Field of the Invention
The present invention relates to a solid-state imaging device in which a first substrate and a second substrate having circuit elements of pixels arranged thereon are electrically connected to each other and an imaging device including the solid-state imaging device.
Description of the Related Art
In recent years, video cameras, electronic still cameras, and the like have generally become widespread. Such cameras employ a charge-coupled device (CCD) type solid-state imaging device or an amplification type solid-state imaging device. The amplification type solid-state imaging device introduces signal charges, which are generated and stored by photoelectric conversion units of pixels on which light is incident, into amplification elements of the pixels and outputs signals, which are amplified by the amplification elements, from the pixels. In the amplification type solid-state imaging device, such pixels are arranged in a two-dimensional matrix shape. Examples of the amplification type solid-state imaging device include a CMOS type solid-state imaging device using complementary metal oxide semiconductor (CMOS) transistors.
In the related art, CMOS type solid-state imaging devices employ a method of sequentially reading out signal charges, which are generated by photoelectric conversion units of pixels arranged in a two-dimensional matrix shape, for each row. In this method, since exposure timings in the photoelectric conversion units of the pixels are determined depending on a start and an end of reading-out of signal charges, the exposure timings differ depending on the rows. Accordingly, when a fast-moving subject is imaged using such a CMOS type solid-state imaging device, distortion of the subject occurs in the captured image.
In order to remove the distortion of a subject, a simultaneous imaging function (global shutter function) to realize simultaneous storage of signal charges has been proposed. The number of applications of CMOS type solid-state imaging devices having a global shutter function is increasing. CMOS type solid-state imaging devices having a global shutter function generally need to include storage capacitors having a light-blocking effect in order to store signal charges generated by the photoelectric conversion units until the signal charges are read out. In such CMOS type solid-state imaging devices, all pixels are simultaneously exposed, signal charges generated by the photoelectric conversion units are simultaneously transferred to the storage capacitors in all the pixels and are temporarily stored therein, and the signal charges are sequentially converted into pixel signals and read out at a predetermined readout timing.
However, in the CMOS type solid-state imaging device having a global shutter function, the photoelectric conversion units and the storage capacitors have to be formed on the same plane of the same substrate and thus an increase in chip area is inevitable. In a waiting period until signal charges stored in the storage capacitors are read out, signal quality degrades due to noise based on light or noise based on a leakage current (dark current) generated in the storage capacitors
A solid-state imaging device in which a first substrate having photoelectric conversion units formed thereon and a second substrate having storage capacitors formed thereon are bonded to each other and the first substrate and the second substrate are electrically connected to each other by microbumps is disclosed in Japanese Unexamined Patent Application, First Publication No. 2013-9294 (hereinafter referred to as Patent Literature 1).
The solid-state imaging device disclosed in Patent Literature 1 is provided with a clamping capacitor for fixing a voltage based on signal charges generated by the photoelectric conversion unit of the first substrate and a sampling transistor (the sampling transistor 108 in Patent Literature 1) for sampling and holding the signal charges in a storage capacitor (the analog memory 110 in Patent Literature 1) of the second substrate. One end (drain terminal or source terminal) of the sampling transistor connected to the clamping capacitor is in a floating state in which the potential is not fixed while the sampling transistor is turned off.
The solid-state imaging device disclosed in Patent Literature 1 is provided with an amplification transistor (first amplification transistor 105 in Patent Literature 1) outputting an amplified signal to the clamping capacitor and a current source (load transistor 106 in Patent Literature 1) constituting a source follower circuit along with the amplification transistor. During a period (readout period) in which the signal charges stored in the storage capacitor are read out, since the current source is turned off, one end of the current source connected to the amplification transistor is in a floating state.